1. Field of the Invention
The present invention relates to a phase comparator with a wide dynamic range that can be used in a phase locked loop.
It can be applied notably to the making of loops for the computing of delta pseudo-distances in satellite radio-navigation receivers.
2. Description of the Prior Art
A phase locked loop is formed, in a standard way, by three elements consisting of a variable frequency local oscillator known by the abbreviations VCO, a phase comparator to discriminate the phase difference between the signal given by the local oscillator and an incident signal, an amplifier and a loop filter controlling the frequency of the local oscillator on the basis of the phase difference given by the phase comparator.
With a loop such as this, the frequency of the oscillator is no longer controlled efficiently either because the level of the incident signal diminishes, when the signal-to-noise ratio is excessively low in the passband defined by the loop filter or because the phase dynamic range of the phase comparator, namely its ability to give a valid signal when the phase difference between the reference signal and the incident signal is great, is exceeded by the phase swings of the signal.
Under these conditions, to obtain high quality measurements on a noise-infested incident signal or on a signal of low value, it is necessary to reduce the passband of the phase locked loop. This is done by increasing the filtering time constant, but then the trailing of the loop at the phase variations between the incident signal and the reference signal is increased and, to prevent a stalling of the loop, the phase comparator should be capable of accepting operation with major phase differences.
However, known phase comparators of the exclusive-OR, tangent or cotangent type limit the phase difference between the incident signal and the reference signal to a few radians. For greater swings, this leads to the use of comparators of non-linear transfer functions, thus greatly complicating the making of the device, and generally causes the loss of the information on phase difference for a limited period of time.
The aim of the invention is to overcome the above-mentioned drawbacks.